It is well known that in industrial scale production of polyethylene, Ti/Mg catalyst systems are in the forefront. Relevant research on the catalysts is focused on the catalytic activity, particle morphology, particle size distribution, hydrogen response, copolymerization performance, and the like. For slurry phase polymerization processes of ethylene, it is quite important that the catalysts used should have higher catalytic activity for controlling the particle size and particle size distribution of the resultant polyethylene polymer. During the polymerization of ethylene, in particular in slurry phase polymerization of ethylene, fine polymer particles readily form. Such fine polymer particles will likely cause the generation of static electricity, leading to the occurrence of “dust” phenomenon, and the formation of agglomerates which may block the transfer conduit systems during after-treatment. The most effective approach for controlling particle size and particle size distribution of the polymer is to control the particle size and particle size distribution of the catalyst used.
In the prior art, two methods are typically used to prepare the main catalyst components in order to obtain catalysts having uniform particle diameter and good particle morphology:
In the first method, powdery main catalyst components are obtained by dissolving a magnesium compound, such as magnesium dichloride, into a solvent to form a homogeneous solution, combining the solution with a titanium compound and optionally with an electron donor compound to precipitate a solid comprising magnesium, titanium, and the optional electron donor compound, followed by treating the solid with excess liquid titanium compound. The main catalyst components are then mixed with co-catalyst component to form catalyst systems. See, for example, the disclosures of CN1099041 A (U.S. Pat. No. 5,459,116), CN1229092 (U.S. Pat. No. 6,617,278B1), CN85100997 (U.S. Pat. No. 4,784,983). There are several drawbacks to such conventional methods. The particle size and the particle size distribution of the catalysts are controlled completely by the precipitation process leading to a preparation with poor stability. There are also serious problems with the recovery system and the environment. In addition, the cost of the catalysts is rather high due to the need to use a large amount of a liquid titanium compound. Moreover, at times, the hydrogen response and catalytic activity of the catalyst systems are unsatisfactory in that it is difficult to control the particle size, resulting polymer powder with a relatively broad particle size distribution.
In the second method, the active component of the catalyst is directly supported on an inert support, such as silica. Since the diameter of the particle size of the silica support is easily controlled with good particle morphology, catalyst particles having uniform particle size can be obtained. However, the loading of the active component on the support is limited, therefore, the titanium content in the catalyst systems obtained by such a method is relatively low with low catalytic activity for polymerization. For instance, CN1268520 discloses a catalyst that is prepared using magnesium dichloride with silica as the support and titanium tetrachloride as the active component. MgCl2 in tetrahydrofuran (THF) is reacted with TiCl4. The reaction product is then combined with a silica treated with aluminum alkyls. The THF is then removed to give a main catalyst component. When the catalyst is used in the polymerization of ethylene, a low level of polymerization was obtained because of the lower titanium content in the catalyst. Although such catalyst systems can be used in gas phase fluidized bed polymerization of ethylene, it is undesirable for use in slurry phase polymerization of ethylene because of the low level of polymerization obtained.
For slurry phase polymerization of ethylene, the catalysts are required to exhibit a high level of catalytic activity, good particle size distribution, and good hydrogen response. Good hydrogen response is indicated when the melt index of the polymer obtained is easily adjusted by varying the partial pressure of hydrogen in the polymerization process to obtain various grades of polyethylene resins. The catalyst systems described provide unsatisfactory levels of hydrogen response.
Therefore, it is desirable to provide a catalyst which exhibits high catalytic activity, with narrow particle size distribution, and provide a good hydrogen response that is suitable for slurry phase polymerization process of ethylene.